Personally, I'm a huge fan of Nautilus. It has tabbed browsing (File->New Tab or CTRL+T), allows some layout customization (particularly if you're willing to edit more advanced settings in dconf), and has a large selection of plugins available. In Ubuntu, you can view the available Nautilus plugins with Aptitude (aptitude search '^nautilus-') or by searching for "nautilus" in Synaptic Package Manager.

If I had to choose another file manager, though, it would definitely be PCManFM.

For me it's Dolphin. Extremely customizable, tabbed/split view browsing, plugins, all that good stuff. No dumbing it down and yet fairly obvious how to work with it. Purty too. Not being GTK based may be a turnoff though, I'm a bit biased towards KDE.

Edit: Forgot Nemo, a fork of Nautilus. The Mint guys weren't happy with where Nautilus was going so they forked it. Brought back some of the removed features, etc.

Although its not my first choice, Dolphin is an excellent file manager! Ultimately, one's choice of file manager comes down to personal preference anyway.

If you want to set Dolphin as your system-wide default file manager, you can edit /usr/share/applications/defaults.list and replace inode/directory=nautilus.desktop with inode/directory=dolphin.desktop, then reboot, of course. (sudo gedit /usr/share/applications/defaults.list is likely the command you want to run to edit that file.)

That's the drawback of using KDE stuff in GTK based desktops, two different things running. It is possible, haven't done this myself in quite a while (I went all KDE myself long ago), and not sure how accurate this still is, but... (comments say it works with Unity as well.)http://www.webupd8.o...-native-in.html